Geological Society, London, Special Publications

Tertiary kinematics in the Lepontine dome

O. Merle, P. R. Cobbold and S. Schmid

Geological Society, London, Special Publications 1989; v. 45; p. 113-134 doi:10.1144/GSL.SP.1989.045.01.06

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© 1989 Geological Society of London Tertiary kinematics in the Lepontine dome

O. Merle, P. R. Cobbold & S. Schmid

SUMMARY: In the Pennine zone of the central (Lepontine) , the mineral-zone boundaries of Tertiary metamorphism and the regional attitudes of foliation and thrust sheets define a dome. We investigate the Tertiary kinematic history of the dome using structural, metamorphic and geochronological data. Thus we distinguish two main ductile deformations, that occurred under differing metamorphic conditions. The first high- temperature deformation (HTD) developed under amphibolite facies conditions. It is responsible for (1) a radiating pattern of stretching lineation trends at the scale of the Lepontine area, and (2) structures indicating overthrusting towards the European fore- land in the lineation direction (i.e. N and NW). The HTD event is attributable to NW-SE Tertiary convergence between Europe and the Adriatic sub-plate. The second retrograde deformation (RD) is responsible for differential uplift of the Pennine zone, with formation of an earlier subdome to the E ( subdome, 25-20 Ma) and a later one to the W (Simplon subdome, 15-10 Ma). In the Ticino subdome, kinematic indicators reveal a horizontal shearing top W. Some major structures such as the Maggia steep zone and the vertical duplication of the Antigorio basement nappe (Bosco-Gurin area) may be related to this westward phase. Dextral transcurrent shearing along the southern steep belt and backthrusting of the central Alps over the southern Alps along the N dipping Insubric mylonite belt are probably coeval with the retrograde deformation recorded in the Ticino subdome. In the Simplon subdome, kinematic indicators reveal a radial motion parallel to the lineation, which plunges towards the SSW in the Verampio window and towards the WSW at the Simplon Line. Serial uplift of Ticino and Simplon subdomes is attributed to continuing dextral transpression at the boundary between Europe and the Adriatic sub-plate. Finally, the back-folding event located at the northern limit of the Pennine zone postdates both HTD and RD deformational events described above. It is part of a later pop-up responsible for uplifting the external crystalline massifs.

Introduction Lepontine dome. First, it occupies a critical position within the Pennine zone, which is often In this paper, we use structural, metamorphic considered to represent an originally thinned and geochronological data to study the Tertiary continental margin (or margins) between the kinematics of the . Many of the European plate proper to the N, and the data we have taken from the abundant literature southern Alps, belonging to an Adriatic sub- dealing with this area, but some structures plate of African affinity, to the S (Fig. 1). In the (stretch lineation patterns and kinematic indi- Lepontine region, the current boundary be- cators) are presented here for the first time. tween the Pennine zone and the southern Alps The Lepontine region appears as a single is the peri-Adriatic fault system, locally known dome in terms of the mineral zone boundaries as the Tonale, Insubric and Canavese Lines (isograds) of Tertiary metamorphism (Niggli (see Schmid et al. this volume). Second, the 1970) and is usually referred to in this sense as eroded Lepontine dome provides a tectonic the Lepontine dome (Fig. 1). The region is also window through the Pennine zone. Third, the a dome, albeit a composite one, on the basis of dome is also at a critical longitude, between the foliation and thrust sheet attitudes (Fig. 2). The eastern Alps, roofed by Austro-Alpine thrust Pennine zone within the dome shows ductile sheets, and the western Alps, which expose deformations of remarkable intensity and com- mostly Upper Pennine units (Monte-Rosa and plexity, coeval with Tertiary metamorphism of Bernhard sheets, Fig. 1). Finally, in this general amphibolite grade and the later cooling history. area, the peri-Adriatic fault system bends round Thus the Lepontine dome provides an excellent from an EW attitude (Tonale line) to an almost and unusual opportunity for studying deep- N-S attitude (southern Canavese line). In such seated processes of deformation (probably as- a context, it is perhaps not surprising that the sociated with plate convergence), followed by internal structure of the Lepontine dome is shallower processes (associated with uplift). complex. Nevertheless, we feel that an under- There are several reasons for studying the standing of it should be helpful towards a

From: COWARD,M. P., DIETRICH, D. & PARK, R. G. (eds), 1989, Alpine Tectonics, II 3 Geological Society Special Publication No. 45, pp. 113-134. II4 O. Merle, P. R. Cobbold & S. Schmid

FIG. 1. Simplified geological map of the Lepontine dome. In the Pennine zone, cover rocks (stippled) are interleaved with slices of crystalline basement (Ad: Adula, An: Antigorio, Bd: Bernhard, CI: Cima-lunga, Le: Leventina, Lu: Lucomagno, Ma: Maggia, MI: Monte Leone, Mo: Moncucco, Mr: Monte Rosa, Se: Sesia, Si: Simano, Su: Suretta, Tb: Tambo,). Also shown are Verampio window (Vr), Berisal synform (Be), Glishorn antiform (GI) and Bergell intrusion (Ber). Periadriatic faults (Canavese, Insubric and Tonale Lines) separate the Pennine zone from the southern Alps, which include the Ivrea zone (Iv). Thick dashed line encloses area of amphibolite facies metamorphism of Tertiary age. Cs indicates the line of the cross section shown in Fig. 9. For numbered localities and peaks see key at bottom fight. broader-scale understanding of Alpine struc- summarize what little is known of its pre- tures and kinematics. Tertiary history. In more detail, we discuss Since this contribution aims at regional petrological and geochronological data on the kinematics, we primarily make use of the large Lepontine metamorphic event. This allows us scale patterns of foliations, lineations and shear to refer to metamorphic assemblages when de- indicators at amphibolite grade, or later green- scribing linear and planar fabrics or kinematic schist grade. As a consequence, the reader indicators. Using the patterns of lineations and familiar with the existing literature will find that the associated senses of shear, we finally attempt we do little justice in this paper to the complexity a Tertiary kinematic interpretation of the of local deformation phases, especially the Lepontine dome. The inferred motions are so geometrical style, orientation data and inter- thoroughly three-dimensional, that we are not ference patterns of folding on all scales (such as yet able to use balanced cross-sections, or any discussed in Huber et al. 1980). This short- other method, to calculate total amounts of coming simply results from the fact that we are displacement. not yet able to fully integrate this kind of struc- tural analysis into a regional kinematic history and that we are not aware of any previous The structural dome attempts to do so. We first describe the general structure of the In the Lepontine region, a geological map of Lepontine dome itself, including the thrust sheet the Pennine zone (Fig. 1) looks much like a sequence and the foliation pattern. Then we sliced onion, for several geometrical reasons. Lepontine dome - Tertiary kinematics I 15

FIG. 2. Current attitude of main foliation in western and central parts of the Lepontine dome. Map at left shows smoothed strike and dip of foliation. Steep dips of 60~ to 90~ are shown by black dot centred on strikeline; moderate dips of 30~ to 60~ by tick on downdip side of strikeline; flattish dips of O ~ to 30~ by lack of symbol. Numbered localities are as in Fig. 1: Biasca (Bi), (Be), Locarno (Lo), Bosco-Gurin (Bg), Domodossola (Do), Brig (Br) and Ilanz (I1). Data from Schmidt & Preiswek (1908), Knup (1958), Milnes (1974b), Steck (1984), Mancktelow (1985), supplemented by the authors' own measurements. Block diagram at right was drawn by computer using true perspective. Platelets are tangent to foliation as estimated at sea level, after regional averaging in areas 1 kmx 1 km square and interpolation using multiquadrics (Barbotin and Cobbold, work in progress).

First, in analogy with the shells of an onion, the (Leventina, Lucomagno), considered infra- region is composed of sheets of Hercynian Pennine by some authors (e.g. Milnes 1974a). basement, typically a few kilometres thick, The overlying Simano sheet dips gently away to interleaved with thinner sheets of Mesozoic the E and W. Further E, the successively higher sediments (Schmidt & Preiswerk 1908, Argand Adula, Tambo and Suretta sheets dip eastward 1911, Staub 1924, Amstutz 1971, Milnes 1974a). by up to 25 ~ on average. They are therefore Early authors described these structures as fold predicted to occur at depths of up to 20 km nappes, but Milnes (1974b) showed that large or so beneath the overlying Austro-Alpine scale folding postdates the emplacement of units which form the eastern boundary of the thrust sheets. Lepontine dome, and are attributed to the Grossly parallel to the sheets themselves (and Adriatic sub-plate. In the SW corner of the to the axial surfaces of major post-nappe iso- region, the uppermost Pennine sheets (Monte- clines) is a composite foliation at amphibolite Rosa and Bernhard units) are separated from grade. We have verified and, where necessary, lower ones by the Simplon Line, which is basi- supplemented the foliation attitudes measured cally a SW dipping normal fault (Mancktelow by Schmidt & Preiswerk (1908), Knup (1958), 1985). This fault also appears to offset the Milnes (1974b), Steck (1984) and Mancktelow metamorphic mineral zones (Fig. 1). In the N (1985). We have then smoothed the data (Fig. and S of the area are steep belts (Milnes 1974a), 2a) and constructed a simple block diagram one bordering the External Crystalline Massifs (Fig. 2b), showing the central and western parts (Aar and Gotthard), the other next to the of the region. Thrust sheets and foliation atti- Insubric Line. Both steep belts are generally tudes form a regional dome. considered to postdate the main deformational To a first approximation, the dome has mirror and metamorphic events (Milnes 1974a, Huber symmetry about a central valley (the Val et al. 1980, Simpson 1982, Steck 1984, Mancel Leventina) running SSE from to & Merle 1987). The southern steep belt is in Bellinzona (Fig. 1). At the bottom of this valley fact the southern limb of an antiformal struc- are exposed the lowermost granitic gneisses ture, most easily visible at the western and II6 O. Merle, P. R. Cobbold & S. Schmid eastern sides of the dome, where the axis metamorphism indicate a geothermal gradient plunges gently to the W or E, respectively. In higher than average. A thickened crust with a the W, this structure, known as the Vanzone low geothermal gradient is probably difficult to antiform, folds the Monte-Rosa and Moncucco deform and so an alternative hypothesis is to units. In the E, the units between the Bergell consider a period between the Cretaceous and intrusion and the Austro-Alpine units are simi- Tertiary events, during which there was little larly folded. Notice that mineral zone bound- deformation. Tertiary deformation due to hori- aries have arcuate appearances in both these zontal forces from the Adriatic sub-plate could corners of the dome. Finally, to complete the perhaps have taken place as the crust became description of the broadly symmetric features of thin enough, by erosion after a late Cretaceous the dome, we notice that the Simplon Line in uplift (see Hurford & Hunziker 1985). the W and the Engadine Line in the E are In any case, the kinematic record of this early symmetrically oriented and have similar pos- stage of crustal collision is now completely itions at the two sides of the dome. overprinted by strong Tertiary deformations As well as these more symmetric features, and metamorphism. the dome has some pronounced departures from mirror symmetry (Fig. 2b). Thus we distinguish the Ticino subdome in the E, separated from Tertiary metamorphism and cooling the Simplon subdome in the W by the N-S trending Maggia steep zone, termed the Numerous petrological and metamorphic stud- 'Querzone' by Preiswerk (1918) on account of ies show that a high-temperature meta- its anomalous attitude. At its SE end, the morphism took place during Tertiary times in Maggia steep zone curves into parallelism with the Pennine zone (Ernst 1973, Frey et al. 1974, the southern steep belt. Immediately SW of the 1980). Mineral assemblages are those of the Maggia steep zone, the foliation traces form a amphibolite grade and mineral zones are nested triangular transitional domain, between the in a concentric pattern on a map (Niggli 1970). Maggia steep zone itself, the southern steep Thus there is a systematic decrease of meta- belt and the NE striking Bosco-Gurin structure morphic grade from southern regions (Locarno (Fig. 2a). Throughout much of the Simplon and Bellinzona area) towards the N, E and W. subdome, the foliation strikes NE whereas in Whereas there is no evidence against a possible the Ticino subdome, it strikes NNW. sub-horizontal attitude of isotherms during the climax of metamorphism (Frey et al. 1980), isograd surfaces are now steeply dipping in the Pre-Tertiary history Simplon subdome, at the eastern margin of the Pennine zone and in the northern steep zone Relict high-pressure mineral assemblages (Streickeisen & Wenk 1974, Fox 1975, (eclogites) are well known from the Sesia zone, Thompson 1976, Klaper & Bucher-Nurminen but have also been described in the central 1987). Thus, at first sight the large scale pattern Alps (Ernst 1976, Evans & Trommsdorf 1978, of isograd surfaces looks like the one expected Heinrich 1982). Pressures and temperatures es- for a true thermal dome (see Turner 1968). timated from garnet and clinopyroxene and However, large scale folding of originally sub- from the stability of jadeite, have a regional NS horizontal isotherms may have occurred after gradient, the pressure varying from 9-13 kb in the climax of metamorphism (Milnes 1975, the N to 20 kb in the S (Heinrich 1982, 1986). Frank 1983). The near coincidence of the struc- Although this metamorphic event is imperfectly tural and metamorphic domes is highly sugges- dated, radiometric data suggest a Cretaceous tive of such a folding. age (Hunziker et al. this volume). The high- The timing of metamorphic events in the pressure metamorphism can be interpreted as Pennine zone is controversial and this in itself resulting from crustal subduction before or dur- suggests that the notion of a simple thermal ing collision between two tectonic plates dome may be misleading. Geochronological (Triimpy 1980, p. 80). For how long did this data from muscovite-phengite (Rb-Sr) at the process continue? One possibility is that em- western and eastern periphery of the Lepontine placement of basement slices continued without dome (Hunziker 1970, Steinitz & J/iger 1981), interruption from the late Cretaceous to the outside the staurolite isograd, yield ages of late Oligocene; but this is probably too simple 38-35 Ma. These ages have been interpreted a history. Thus, the P-T conditions of eclogite as the peak of Lepontine metamorphism (J~iger facies indicate a geothermal gradient lower than 1973). Following this interpretation, the average, whereas the P-T conditions of Tertiary younger ages inside the straurolite isograd all Lepontine dome - Tertiary kinematics I I7 have to be considered as cooling ages. This view Cooling of the Lepontine area has tradition- assumes that there is a single peak in the time- ally been attributed to 'uplift', without further temperature curve (referred to as the 'Lepontine explanations of its causes. The use of fission event') and according to this concept the min- track data provided a major breakthrough in eral isograds would approximate an isotherm at specifying the cooling history. For a region N of one and the same instant in time. The ages of Locarno, Hurford (1986) demonstrated a period 38-35 Ma are identical with those obtained of very rapid cooling between 23 and 19 Ma, for the greenschist metamorphic event in the followed by slower cooling rates compatible western Alps (Bocquet et al. 1974, Frey et al. with current uplift rates (Fig. 3, see also Werner 1974, Chopin & Malusky 1980, Chopin & Moni6 et al. 1976, Steiner 1984). Since the formation 1984). of the southern steep belt appears to be in- In contrast, ages obtained within the timately related to backthrusting along the Lepontine dome using the U-Pb method on Insubric Line, the period of rapid cooling is best monazites or K-Ar on amphiboles are much attributed to uplift caused by backthrusting and more recent (around 25-20 Ma). Hence some backfolding during an Insubric phase (Argand workers have reappraised the chronology of the 1916), of Oligocene-Miocene age and contem- Alpine metamorphism, claiming that conditions poraneous with northwards thrusting in the sufficient to produce high grade assemblages Helvetic nappes (Schmid et al. 1987). In this were sustained until around 25-20 Ma (H~inny interpretation, the emplacement of the et al. 1975, K6ppel & Griinenfelder 1978, Lepontine area into shallow levels of the Earth's K6ppel et al. 1980, Deutsch & Steiger 1985, crust is the result neither of isostatic uplift nor Moni6 1985). of underplating and crustal extension (Platt Furthermore, structural investigations (e.g. 1986), but of a Neo-Alpine compressional Thakur 1973, Huber et al. 1980, Klaper 1982) phase. suggest that more than one episode of mineral The geochronological data also indicate that growth occurred in the Lepontine area with cooling occurred earlier in the E (Ticino sub- respect to established deformation phases. Ad- dome) and later in the W (Simplon subdome) mittedly these deformation phases do not (J~iger et al. 1967, Wagner et al. 1977). This has need to be contemporaneous all over the area led to a model of differential uplift within the considered. However, Rb-Sr ages on micas Pennine zone (Bradbury & Nolen-Hoeksema (Steiger & Bucher 1978, Steiger 1983) more 1985). Westward propagation of uplift may be directly suggest a second episode of mineral due to dextral transpression in front of a rigid growth, particularly in the northern part of the indenter as, for example, the Adriatic sub- Lepontine dome, S of the Gotthard Massif. plate with the Ivrea zone at its NW corner This mineral growth does not necessarily indi- (Schmid et al. this volume). cate a second peak in temperature, but may indicate that mineral isograds did not form at the same time all over the Lepontine area. Thus, the age of the so-called Lepontine metamorphic event remains unclear, possibly T (*c} because the subject has been considered in terms 700 that are too simple. We suggest that the notion 600 of a single metamorphic event, synonymous 500 with a single period of mineral growth and a :1':':'>:':-:-:':':':':~:" 400 :.;.:.1.:.1.1.1.1-:,1.>101-I single peak in the temperature-time curve SO0 throughout the entire Lepontine dome, should i.>l.l.:.>:.>:':'i-l': ll 200 ~:::V~::~ be abandoned in favour of a more complex and ':':':':'1':':':','.'.'-'~'.1 ~ ~00 9.:.:.:,:.:.:.:.:-:.:-:0:. I dynamic thermal history, which has yet to be -:.1,1-:.:.:.:.:-:.:-:.:.--':1...... 9 t(Ma) l worked out for particular domains within the 35 30 25 20 15 10 Lepontine area. The complexity is due in part to very important post-38-35 Ma events, such FIG. 3. History of cooling and deformation in the as the intrusion of the Bergell batholith (30 Ma, Lepontine dome. Curve gives temperature (T) as a K6ppel & Griinenfelder 1975), an Oligocene function of time (t) for the last 35 Ma according to model B (II) of Hurford (1986), applicable to the phase possibly related to the intrusions along Maggia valley only. There, high-temperature the Insubric Line (Laubscher 1983) and parti- deformation (HTD) ended at about 25 Ma and was cularly the subsequent formation of the southern followed by retrograde deformation (RD). Within and northern steep belts (Hurford 1986, Schmid the Simplon subdome, rapid cooling started later than et al. 1987, Merle 1987). 25 Ma ago (see Schmid et al. this volume, Fig. 3). II8 O. Merle, P. R. Cobbold & S. Schmid

the peak of metamorphism in this particular High-temperature deformation area (Merle et al. 1986). (HTD) We have measured the attitude of HTD stretch lineations at 882 localities throughout In this section, we refer to deformation that the Lepontine dome (Fig. 4), thus confirming occurred before the onset of rapid cooling. Pro- the unusual radiating pattern first described by visionally, we consider the HTD throughout Wenk (1955). If we compare the attitude of most of the Pennine zone to have occurred foliation and lineation (see Figs 1, 2 and 4), we between 40 and 25 Ma. This age bracket is see that in the central areas of fiat-lying foli- consistent with geochronological data discussed ation (Ticino and Simplon subdomes), the lin- in the previous section and with the culmination eations fan out in directions approximately of Meso-Alpine shortening in the late Eocene perpendicular to the peri-Adriatic fault system. to early Oligocene (38-33 Ma) according to Even in the anomalous Maggia steep zone, this several authors (e.g. Ayrton & Ramsay 1974, radial pattern of fiat-lying lineations is not Milnes 1978, Hsii 1979, Triimpy 1980, p. 90). disturbed, the lineations following the strike of Because of the long period of high temperature the steep zone. (40-25 Ma) we cannot further constrain the As one approaches the northern steep belt age of the HTD using metamorphic arguments. near Airolo, both lineation and foliation We cannot even exclude the possibility that acquire steep attitudes, suggesting that they some areas of the Lepontine dome inside the have become folded together about a horizontal staurolite isograd reached amphibolite grade E-W axis during later events. Similar geo- conditions previous to the event dated at around metrical relationships occur as one approaches 38-35 Ma outside this isograd. Furthermore, the southern steep belt, either in the Bellinzona although the onset of rapid cooling is well dated area and further E, where the lineation is nearly in the Maggia area at 25 Ma (Krppel & vertical, or in the Domodossola area, where it Griinenfelder 1975, Werner et al. 1976, Steiner now pitches at about 45~ This suggests that 1984, Hurford 1986), some areas to the N (S of foliation and lineation were folded together the Gotthard Massif) and W (Simplon domain) about horizontal axes during a later event in the were at high temperatures considerably later southern steep belt as well. However the situ- than 25 Ma ago (J/iger et al. 1967, Purdy & ation is not as simple as this, because for about J~iger 1976, Wagner et al. 1977). Thus we do 50 km between Domodossola and Bellinzona, not consider the HTD to be a well-timed tectonic passing through Locarno, the HTD lineation is event, synchronous over the entire area. fiat-lying, often pitching gently E, but some- times even gently W. If the foliation is restored to the horizontal by rotating about the E-W strike, the lineation remains E-W; whereas im- Stretch lineation mediately to the N, in the fiat-lying area, it The main lineation observed in the Pennine currently strikes NW. We therefore conclude zone is a preferred orientation of grain aggre- that the steep belt between Domodossola and gates, individual grains, fold axes, rods and Bellinzona cannot have undergone rigid ro- boudins. This structure seems to track the di- tation alone during later events. Either the HTD rection of maximum total stretch. Fold axes lineation formed in a fiat-lying E-W attitude, parallel to the stretch lineation occur at small to or it became reorientated during later events as intermediate scales (Wenk 1955). Occasional a result of superimposed ductile deformation eye structures are probably tranverse sections (strain). This matter is further discussed in the through sheath folds. We have also observed following section. definite sheath folds in oblique sections (Cobbold 1980). Optical observations in thin section show that Kinematic indicators the main foliation and lineation are defined In our general experience, kinematic indicators by mineral assemblages of amphibolite grade, are less clear and abundant in rocks of amphi- such as quartz+plagioclase(An>17)+biotite+ bolite grade than they are in rocks of greenschist staurolite+kyanite+garnet. Thin sections fre- grade. Nevertheless we have found high- quently show shear bands associated with the temperature shear indicators in the Lepontine growth of metamorphic minerals. A study of dome, both at outcrop scale (Fig. 5a) and in inclusions within garnets from the Simplon area thin section (Figs 5b, c and d). In deformed indicates that growth of garnet is contempor- granites, we mainly used the relative attitudes aneous with rising temperature and predates of foliation and shear bands with amphibolite Lepontine dome - Tertiary kinematics I I9

PLUNGE OF LINEATIONS '~.,,~, 6~,., ![ t~,~

lOkm I I

A ~3 , HTD lineotion.,

--. .-..~ --.. ~, ..-.q~- "~Z~ HTD + RD ...... ~'~ ~ ~ ~-= '~, lineotions

TONALE LINE

..../ v

H T D LINEATIONS

FIG. 4. Current attitude of high-temperature stretch lineations in central and western parts of Lepontine dome (new data only). Each arrow represents a single measurement. Length of arrow is inversely proportional to plunge in degrees (see key). Solid curves are a selection from the geological boundaries of Fig. 1. Numbered localities are as in Fig. 1. The age of the lineations in the southern steep belt (south of the solid broken line) cannot always be inferred with certainty: there are HTD lineations rotated by the backfolding as well as lineations formed during the RD event. For lineations within the Insubric mylonite belt see Schmid et al. (1987) (data from the Monte-Rosa area by E. Barbotin). grade assemblages (C/S structures of Berth6 et In the northern steep belt around Airolo, al. 1979), as well as the sigmoidal shapes of HTD shear indicators appear to have become recrystallized tails around old grains (Simpson folded into an upright position, together with & Schmid 1983, Passchier & Simpson 1986). foliation and lineation. Restored to a horizontal Such structures, together with asymmetric folds attitude, by rigid rotations about E-W axes, of the foliation, are especially clear in the the shear indicators have top to N senses, as m Leventina orthogneiss (Fig. 5). In paragneisses the centre of the Ticino subdome. Further evi- we had greater difficulty in determining shear dence for such rotations will be discussed later. senses, but sometimes, like Mawer (1987), we In the southern steep belt, HTD shear indi- were able to use shear bands, asymmetric folds, cators have been found so far at two localities rolling structures or rotated boudins (Van Den (Fig. 6). One is in the Monte-Rosa unit at Driessche & Brun 1987). Villadossola, near Domodossola (Schmid et al. In the flat-lying areas of the Lepontine dome, 1987); the other is in the Monccucco unit at HTD shear senses are consistently top to N to Ponte Brolla, near Locarno. Both yield dextral WNW in the lineation direction (Fig. 6). In strike-slip shear senses in almost perfectly hori- most areas, there is little or no retrograde over- zontal directions. Were these horizontal shear print, so that these shear directions appear to directions operative during the HTD event, or be original ones, operative during the HTD have the structures been rotated? If foliation is event. An exception is the Simplon area, where restored to the horizontal, by rigid rotation there is a retrograde overprint, discussed in a about strike, the shear senses become top to W, later section. compatible in sense, but not in direction, with x2o O. Merle, P. R. Cobbold & S. Schmid

FIG. 5. Kinematic indicators for high-temperature deformation. (a) Shear bands and sigmoidal porphyroclasts in the flat-lying Leventina gneiss. Photograph of vertical outcrop (N at left), scale bar 5 cm long. (b) C-S structures and folds in the fiat-lying Leventina gneiss (thin section, vertical, N at left, scale bar 2.5 cm long). (c) and (d) Detail from (b), showing shear bands of top to N sense, containing assemblages of two micas, feldspar and quartz. Dynamic recrystallization of potash feldspar (c) and dynamic recrystallization of quartz by grain boundary migration (d) indicate high temperatures during formation of shear bands. Scale bars 0.83 cm long (c) and 0.20 mm long (d). Lepontine dome - Tertiary kinematics 121

the top to NW shears observed in the flat-lying southern steep belt. At the Ponte-Brolla lo- areas immediately to the N. Thus a component cality, shear indicators (shear bands and asym- of rigid rotation during later events is not im- metric folds and boudins) are coeval with the possible; but it does not appear to be sufficient beginning of anatexis (Gapais, personal com- in itself to account for the present attitudes, as munication, 1987). The rocks are migmatitic discussed in detail in the previous section on and there is no evidence of a retrograde ductile HTD lineations. We conclude that there has overprint. We therefore conclude that dextral been additional ductile strain in this part of the strike-slip at Ponte-Brolla occurred at high 122 O. Merle, P. R. Cobbold & S. Schmid

KINEMATIC INDICATORS

*'~1 HTD SHEAR SENSE ~ ~0 krn~

~-k~.~ ~ \ TONALE LINE

FIG. 6. Shear senses in the Lepontine dome. Each arrow represents a sense of shear, determined from a population of kinematic indicators at outcrop and in thin section, where such a population gave consistent results at a scale of about 100 m. In central areas of fiat-lying foliation, horizontal shear components are shown by single arrows representing motion of roof (large black arrows for HTD, wide empty arrows for RD in Ticino subdome, small black arrows for RD in Simplon subdome). Note the reversed sense of shear in the Berisal and Glishorn backfolds. In the southern steep belt and along the Engadine Line, wrench components are shown by zigzag arrows (full black arrows for HTD, empty arrows for RID). In the northern steep belt, asterisks show localities where HTD shear directions are currently vertical (for interpretation, see Fig. 10). East of dashed line, shear-sense indicators have been observed recently at outcrop (by P. R. Cobbold, M. Ballevre, D. Gapais and E. Le Goff), but not yet in thin section, so that metamorphic conditions are not yet reliably known. temperature. Unfortunately we do not know Merle 1987). Further E, the RD is only oc- whether it occurred in the age bracket of 40-25 casionally associated with syntectonic growth of Ma, or at the beginning of the Insubric phase. chlorite. From studies of the RD, we infer different kinematic histories in the Simplon and Ticino subdomes. The southern and northern steep belts also have rather special histories. Retrograde deformations (RD) Southern steep belt In this section, we refer to deformations coeval with the cooling of the Lepontine area, The formation of the southern steep belt has i.e. from 23 Ma to 19 Ma in the Ticino subdome been shown to be contemporaneous with (Hurford 1986), and significantly later in mylonitization under greenschist facies con- the Simplon subdome (Bradbury & Nolen- ditions along the Insubric Line (Zingg & Schmid Hoeksema 1985). 1983, Schmid et al. 1987), during the Insubric Geological mapping of stretch lineations (Fig. phase in the sense of Argand (1916). Because 7), shear indicators (Fig. 8a, b) and other struc- the Insubric mylonites formed at the interface tures reveal late ductile deformations (Steck between the warm Lepontine domain and the 1984) superimposed under retrograde meta- earlier cooled southern Alps (>500~ vs. 150~ morphic conditions upon the HTD (Merle 1987, at 25 Ma, see Hurford 1986, fig. 7), this retro- Merle et al. 1986). The closing stages of retro- grade overprint is particularly obvious within grade deformation (RD) demonstrably occurred the mylonite belt, which simply represents the under greenschist facies conditions in the southern margin of the steep belt. In the SW of Simplon area (Mancktelow 1985, Mancel & the region, the central Alps were backthrust Lepontine dome - Tertiary kinematics I23

o

~ 2az

~ .

m

o;..~ 124 O. Merle, P. R. Cobbold & S. Schmid

FIG. 8. Kinematic indicators for retrograde deformation. (a) Shear bands at contact between Simano nappe and Leventina gneiss (vertical thin section, E at right, scale bar 1 cm long, see Fig. 9 for locality). Sense of shear is top to W. (b) Shear bands and fold in Baceno schist, Verampio window (vertical thin section, SSW at left, scale bar 2 cm long). Sense of shear is top to SSW. Chlorite content about 20% (c) Shear bands within a quartzitic gneiss near base of Bosco-Gurin thrust (see Fig. 9 for locality). Microstructure of coarse-grained quartz outside a shear band (left) indicates migration recrystallization typical of high temperature deformation. Within same shear band (right), serrate grain boundaries and incipient recrystallization by mechanism of subgrain rotation indicate lower temperatures. Since shear bands represent late instabilities along a continuous strain path, we deduce that shearing occurred while rocks rapidly cooled (compare Fig. 3). Scale bars 0.83 mm long (left) and 0.25 mm long (right). (d) Flat-lying shear zone in Antigorio granite, Lago d'Agaro (locality 8, Fig. 1). Vertical outcrop, SSW at left, hammer gives scale. Shear sense is top to SSW. Lepontine dome - Tertiary kinematics I25

over the southern Alps along the N-dipping established locally, both events are best thought Insubric mylonite belt, as demonstrated by small of in terms of a continuous process of dextral scale shear criteria and steep stretch direc- transpression on a larger scale (Schmid et al. tions (Heitzmann 1987b, Schmid et al. 1987). this volume). Only during the very latest stages Kinematic studies indicate that backthrusting along the Insubric Line (cataclastic deformation was followed by dextral shearing in these mylo- at the Tonale and Centovalli lines, greenschist nites. Although this sequence of events can be facies mylonitization of Lepontine marble 126 O. Merle, P. R. Cobbold & S. Schmid

Laubscher 1971, Fumasoli 1974, Heitzmann recorded by geochronological data. 1987a, Schmid et al. 1987) did oblique conver- Westward shearing may be a key for under- gence across the belt change into almost pure standing major structures such as the Maggia strike-slip. steep zone and at the duplication of the Because of the existing N-S temperature Antigorio granite in the Bosco-Gurin area. An gradient during the Insubric phase, it is danger- E-W cross section through the Maggia steep ous to use ambient temperatures to decide on zone (Fig. 9) shows a large fold overturned the relative ages of deformation. The retrograde towards the W (see Merle & Le Gal 1988 for nature of movements N of the mylonite belt is a fuller description). This structure appears to not always obvious, since higher temperatures fold the earlier high-temperature foliation. prevailed for a longer period of time in the Retrograde mylonite zones and later brittle northern parts of the Lepontine area (Hurford faults all show top to W senses of shear or slip. 1986). In the Valle d'Ossola section, dextral We therefore interpret the overall structure as strike-slip (Monte-Rosa nappe at Villadossola being modified, if not created, by top-W shear- near Domodossola, Schmid et al. 1987) and ing during the RD event. backfolding (Vanzone antiform, Klein 1978, In the Bosco-Gurin area, the Antigorio Milnes et al. 1981) can be attributed to this granite is duplicated (Figs 1 and 9). There is Insubric phase. Dextral shearing occurred under a well developed RD stretch lineation strik- retrograde conditions all along the southern ing E-W, parallel to the fold axis of the steep belt (Fig. 6 and Heitzmann 1987b), Wandfluhhorn fold (Hunziker 1966, Hall 1972, but also under high-temperature conditions see Wandfluhhorn peak located on Fig. 1). In (Villadossola and Ponte Brolla localities). The the core of the upper Antigorio sheet, strain is time relationships between dextral shearing and homogeneous and shear indicators are lacking, backfolding are not established yet and it is except at the base, where the granite, in contact likely that both components of motion were with underlying paragneisses (Bosco-Gurin synchronous in many parts of the southern steep unit), becomes mylonitic in a thin zone dipping belt, as postulated for the lnsubric mylonites. gently E. The sense of shear is top to W. Thus, In any case, the vertical uplift associated with the duplication of the Antigorio granite may be backthrusting appears to have reached a max- due to overthrusting towards the W (Merle & imum between Locarno and Bellinzona. Le Gal 1988). The northern limit of the Bosco- Gurin thrust remains to be recognized in the core of the Antigorio granite itself, but the Ticino subdome Wandfluhhorn fold may be a ductile termi- In the W, from Biasca to Bosco-Gurin, the nation of the Bosco-Gurin thrust. To the S, Ticino subdome displays a set of RD stretch next to the southern steep belt, the Antigorio lineations of nearly EW trend, superimposed granite is refolded in a synformal structure (the on the HTD stretch lineations (Fig. 7). This Masera synform). On a map, the hinge of this second deformation is scarce at the scale of the fold curves in a manner consistent with a dextral subdome, but where it occurs it is usually associ- sense of shear in the southern steep belt. ated with fiat-lying ductile mylonites. Thus, The westward deformation in the Ticino sub- mylonites at least 10 m thick occur at the contact dome is considered to be coeval with the for- between the Leventina gneiss and the Simano mation of the southern steep belt. We believe sheet (Swiss National Grid 706.2-141.45) and that both events occurred at the western and between the Antigorio granite and Bosco- southern margin of the uplifting Ticino subdome Gurin units (SNG 678.3-128.8). Shear senses between 25 and 20 Ma, a period of rapid cool- determined at outcrop or from thin sections ing in the Ticino subdome according to geo- (Fig. 8a) are all top to W. chronological data (e.g. Werner et al. 1976, The retrograde character of this deformation Wagner et al. 1977, Hurford 1986) (Fig. 3). is apparent in thin section. Chlorite has grown On the eastern side of the Ticino subdome, preferentially within shear bands that postdate within the Adula unit, but still within the limit the growth of amphibolite-grade porphyro- of Tertiary amphibolite-facies metamorphism, blasts. However, we have never observed com- retrograde shear indicators with normal-fault pletely retrograde assemblages (as described sense (top to ENE) have recently been observed later for the Verampio window or the Simplon (Fig. 6). The metamorphic conditions of their line). Quartz microstructures within microscale formation and their age relationships with the shear bands often indicate progressive defor- shear indicators in the western part of the dome mation at decreasing temperatures (Fig. 8c) and are not yet clearly determined. this can probably be correlated with the cooling Finally, in the steep zone that prolongs the Lepontine dome - Tertiary kinematics I2 7

VAL ANTIGORIO VALLE MAGGIA VAL VERZASCA VAL LEVENTINA (BACENO) (SOMEO) (BRIONE) (BIASCA) W F~g.sc F~g.8~ E

LEVENTINA gneiss \ ~ SIMANO noppe

~--~ ANTIGORIOnQppe

8OSCO- GURIN unit L- 1Okra_, I

~ MAGGIA nappe

FIG. 9. Schematic geological cross-section running EW from Val Leventina to Val Antigorio across Maggia steep zone and Bosco-Gurin thrust (after Merle & Le Gal 1988). Section line is shown in Fig. 1. The Wandfluhhorn fold cannot be seen as its axis is parallel to the cross section. Arrows show shear senses in retrograde mylonites (see Figs 6, 8a, c).

Engadine Line to the SW (Fig. 1), as well as in area of retrograde deformation beyond the the northernmost exposures of the Bergell in- Simplon Line is not yet known (Merle 1987, trusion, retrograde shear indicators reveal left- Mancel & Merle 1987, Steck 1987). The sense lateral wrench components along strike (Fig. of shear, demonstrated by C/S structures, shear 6). Once again, the metamorphic conditions of bands and asymmetric pressure shadows and formation are not yet known, but the structures porphyroclasts, is top down and SSW in the postdate the Bergell intrusion (30 Ma). Verampio window, top down and WSW at the Simplon Line (Fig. 6) (Merle 1987, Mancel Simplon subdome & Merle 1987). We correlate this defor- mation with the Simplon phase described by Throughout a well-defined triangular area be- Mancktelow (1985, 1987) along the Simplon tween the Simplon Line and the northern lobe Line. At a few localities in the core and front of of the Maggia unit, later flat-lying foliations and the Antigorio granite nappe, the shape of quartz lineations partially overprint older HTD struc- aggregates indicates that the strain ellipsoid is tures (Steck 1984). The second deformation oblate (Greco 1985, Daniel 1986). This strain is occurred under retrograde conditions, at tem- due to the retrograde deformation alone, high- peratures lower than in the Ticino subdome temperature aggregates having been observed (Merle 1987). only at the base and rear of the granite sheet. The stretching lineation in the field is mainly We suggest that the oblate strain is due to defined by an alignment of quartz aggregates, centrifugal radial motion along the divergent muscovite and chlorite. Coeval shear bands RD trajectories of Fig. 7. The strain intensity postdate the growth of garnet, staurolite or appears to increase towards the Simplon Line, kyanite porphyroblasts; chlorite aggregates are but is also very high in some ductile units usually located within microscopic shear bands. (Baceno schist) exposed in the Verampio win- Thin sections of samples from the Verampio dow (Merle 1987). window and the Simplon line indicate P-T con- Folds with axes nearly perpendicular to ditions of greenschist facies (Fig. 8b). Shear the stretch direction and asymmetries con- bands developed within the domain of stability sistent with the bulk sense of shearing are fre- of biotite but generally were contemporaneous quently observed in the Simplon subdome. with the breakdown of this mineral. Non-cylindrical folds can sometimes be seen, The RD lineations form a consistent, slightly but the strain is not strong enough for complete radiating pattern (Fig. 7), restricted to an area reorientation of fold axes into the stretch of nearly flat-lying units, surrounded by zones direction. of steeper attitude (including the Bosco-Gurin The RD in the Simplon subdome must have structure). Towards the W, the extent of this occurred later than about 15 Ma, an age which I28 O. Merle, P. R. Cobbold & S. Schmid represents the end of the amphibolite grade in that the HTD indicators were rotated into an this area (J~iger et al. 1967, Purdy & J~iger 1976, upright position during backfolding. Backfolds Wagner et al. 1977). This age is consistent with postdate well-developed biotites which have the time bracket of 19-6 Ma suggested by been dated at 15 Ma (Steiger & Bucher 1978, Mancktelow (1985) for the deformation re- Ramsay in Steck et al. 1979). corded along the Simplon Line. On the basis North of the Simplon subdome, the senses of of young ages (Frank & Stettler 1979) in fine shear associated with the retrograde stretching fractions of mica, the deformation along the lineations (depicted in Fig. 6) are reversed be- Simplon Line may indeed have lasted until tween the Berisal synform and the Glishorn 9 Ma. Thus, it is likely that the Simplon phase antiform (Fig. 1) (Mancel & Merle 1987, postdated the formation of the southern steep Fig. 8). This reversal is interpreted to be not belt and the RD deformation in the Ticino original, but due to backfolding about axes subdome (20-25 Ma, see discussion in the near-parallel to the earlier retrograde lineations. previous section). Hence, a very young age of less than 10 Ma is inferred for backfolding within the northern steep belt. Northern steep belt The northern steep belt is a region of backfoid- ing located at the boundary between the Pennine Kinematic history zone and the external crystalline massifs (Huber et al. 1980). The backfolds have been described Using the structural, metamorphic and geo- as late structures postdating the high-grade chronological data presented or discussed metamorphism in the Pennine zone (Higgins above, we now interpret the kinematics of the 1964, Chadwick 1968, Cobbold 1969, Thakur Lepontine dome in three stages, which we con- 1973, Huber et al. 1980, Simpson 1982). sider in reverse order (younger to older): The chronological relationships between high-temperature deformation (HTD) and (1) During the youngest stage (about 10 Ma to backfolds are especially clear N of the Ticino present), the Lepontine dome appears to subdome (Airolo area and Lucomagno section). have been rigid, except at its margins. In Small scale HTD shear indicators (C/S struc- the northern steep belt between Brig and tures in the Tremola series of the southern Ilanz, ductile deformation was expressed as Gotthard Massif) show an apparent uplift of backfoiding and southwards overthrusting southern units with respect to northern units (Fig. 10). Ductile deformation also occurred along the vertical stretch lineation (Fig. 10). in the external zones to the N and is respon- Low-temperature flat-lying shear bands and sible for the N6OE striking foliation of the shear zones (top to S) postdate the HTD indi- crystalline massifs (Choukroune & Gapais cators and are associated with the backfolding 1983, Marquer & Gapais 1985). To the S of event (Fig. 10). They become prominent in the the Lepontine dome, dextral strike-slip orthogneiss of the Gotthard Massif which seems motions occurred on the Insubric and to have been less affected by the HTD defor- Tonale faults. All these ductile and brittle mation (see Choukroune & Gapais 1983, motions we attribute to dextral trans- Marquer & Gapais 1985). Thus we conclude pression across the Insubric Line.

" back Fo/,

FIG. 10. Schematic N-S section through northern steep belt (Airolo and Lucomagno area, see Fig. 1), illustrating shear indicators (large hollow arrows) of earlier HTD event (1), fiat-lying in the S, but steepened and overturned in the N, as a result of later back-folding event (2). Late retrograde shear bands (small arrows) are associated with backfolding. Lepontine dome - Tertiary kinematics I2 9

W 0 0 -'-'~ ~ o ~

.@ ~ I= w 8 0 { _~o Ii r ~-a I

t ~.~-~ ~ ~

Z i~ ~0 ~N I I =l 0 ,-,~ 0 (.On J~Z' ~ ~oeS~> o<,. 8 <: or) t"-,I 0~"o 0 i ~l ~1~ ~ ~ "7 ~I ~ ,.~ "~ ~, ~

Z =~ ~.~ DO0 (DD /,/ ua o-r f Oo o 0 /i1' ~D ~.~ -~ ~ m ~<0'*~ ~.~~ l,J-I 0 > }1 . o.-.= u-I N Ii/'

z ~ = <', ~.,.= O~ ~__"z ._ <~~>~ ~> ~,~ ~.~i~

,.<,_.,~ o~ %\ ,>, ~oo~o_=

', %. .,~-~ I ,, k\'_~',x, .~ ~ ~ ~ .~ ~ "4-"-- ,, ,,,"' I +-",,,,. ik~ + ~ ~_=~~'o~' =~o~ ~ >:":o

r ,..El ~J .~ I3o O. Merle, P. R. Cobbold & S. Schmid

(2) The Insubric phase (about 25-10 Ma) is tween Europe and the Adriatic sub-plate. best considered as twofold (Fig. 11). The The general direction of this convergence kinematics are closely related in time to the would appear to have been NW-SE, as uplift history of the Lepontine dome. If we during the later Insubric phase; but because attribute the back-thrusting along the motion was roughly radial, centrifugal southern steep belt to the converging motion of the hangingwall would have led Adriatic sub-plate, it seems sensible to ex- to arc-parallel extension. Radial motions of pect some lateral escape of rocks within the this kind have been described in exper- Pennine zone at the same time, especially iments scaled for gravity (Davy & Cobbold at releasing intersections of conjugate in press). In such experiments, the hanging- strike-slip faults such as the Engadine and wall deforms more readily than the footwall. Insubric Lines. The two regional subdomes If we apply these ideas to the Pennine zone, of the Pennine zone (see the map of foliation we infer arc-parallel extension at upper attitude, Fig. 2a) could be due to these levels. Oblate mineral aggregates of amphi- major retrograde deformations,.coeval with bolite grade, frequently observed in fiat- uplift, whose importance seems to have lying areas of the Ticino subdome, suggest been underestimated until now. An earlier that arc-parallel extension may indeed have dome (Ticino dome, 25-20 Ma) to the E operated at this stage, preventing strong and a later one (Simplon dome, 15-10 Ma) crustal thickening and perhaps facilitating to the W of the Pennine zone might thus be initial uplift of the Lepontine dome. The attributed to continuing dextral trans- Insubric and Engadine Line may also have pression at the boundary between both been active at this early stage, their releas- tectonic plates (Fig. 11). The domes could ing intersection facilitating emplacement of have formed in an area of excess shortening, the Bergell intrusion some 30 Ma ago (see located in front of the indenting Adriatic Triimpy 1980, p. 77). sub-plate and at the restraining intersection To conclude, all three stages in the Tertiary of the Insubric and Engadine strike-slip kinematic history inferred for the Lepontine fault zones, whereas horizontal extensions dome can be attributed to dextral transpression were concentrated at the releasing intersec- across the Insubric and Tonale Lines. Uplift of tions to the E and W of the domes (Fig. 11). the dome was mainly due to backthrusting along The NE-SW direction of extension in the the southern steep belt, but was aided by lateral Simplon subdome is kinematically compa- escape of rocks and an associated component tible with NW-SE shortening and dextral of horizontal regional extension within the shearing. The radial stretch trajectories ac- Lepontine area. companying the extension in the Simplon subdome are perhaps attributable to strong gravitational effects (i.e. a spreading pro- cess) contributing to unroofing of the sub- Concluding remarks dome. Physical and numerical models of such gravitational processes character- From our structural study of the Lepontine istically show (a) divergence of stretch area, we conclude that the central Alps during trajectories from topographic highs (b) Tertiary times underwent a tectonic history oblate strain ellipsoids resulting from a rather different from those usually described combination of divergent horizontal shear- for full frontal collision. Whereas the western ing and vertical shortening and (c) strain Alps appear to have undergone frontal collision intensities that increase downslope (see with the Adriatic subplate (Menard & Hudleston 1983, Gilbert & Merle 1987). Thouvenot 1984), the central Alps were sub- (3) Before about 25 Ma, the Lepontine area jected to dextral transpression, with relatively underwent radial shearing (top to N or little crustal thickening. This study also suggests NW). Restoration of the younger back- that post-collisional late Oligocene and early thrusting along the Insubric Line suggests Miocene deformations have so far been under- that the major tectonic units were southerly estimated in the Lepontine area. dipping before backthrusting. If so, the A reconstruction of the main tectonic features radial motion implies an overthrusting to in N- S section cannot show the effects of strike- the N and NW at a level deep enough to slip motion and lateral escape, but it does indi- sustain amphibolite facies metamorphism. cate that backthrusting of the Pennine zone Thus the high-temperature deformation is over the southern Alps is one of the main attributable to Tertiary convergence be- features of central Alpine history during the Lepontine dome - Tertiary kinematics 13I

Tertiary (see, e.g. Fig. 8 in Schmid et al. this ACKNOWLEDGEMENTS" This work was started by the volume, and fig. 3 in Milnes 1978). Taking Rennes group in 1979. At the time, Martin Huber into account the northwestward overthrust- and John Ramsay very kindly provided copies of ing in the external domain, the history of the maps, theses and other publications difficult to obtain Pennine zone at this time is that of an erosional outside . The authors have benefited from useful discussions with colleagues at Ziirich (especially pop-up, which is probably the best model for Geoff Milnes and Nell Mancktelow), Basel (Andr6 explaining the rapid cooling recorded at around Zingg), Bern (Johannes Hunziker) and Rennes 25-20 Ma and the emplacement of amphibolites (Pierre Choukroune, Michel Ballevre and Philippe into shallow levels of the Earth's crust. Davy). Geoff Milnes and John Platt provided careful During the closing stages of deformation, a and constructive reviews. similar but smaller pop-up uplifted the external massifs and formed the northern steep belt, with associated minor backthrusting in the northern Pennine zone.

References

AMSTUTZ, 1971. Carte tectonique de l'Ossola aux developed by bulk inhomogeneous flattening. . Archives des Sciences de Gendve, 24, Journal of Structural Geology, 5, 1 - 10. 5-9. COBBOLD, P. R. 1969. Geology of the Val Camadra - ARGAND, E. 1911. Les nappes de recouvrement des Greina area, Ticino, Switzerland. Unpublished Alpes Pennines et leurs prolongements struc- BSc Thesis, Imperial College, London. turaux. Beitriige zur Geologischen Karte der -- 1980. Sheath folds and large strains in rocks. In: Schweiz, 31, 1-26. Large strain deformation: report on a tectonic -- 1916. Sur l'arc des Alpes Occidentales. Eclogae studies group Meeting held at Imperial College Geologicae Helvetiae, 14, 145-204. on 14 November 1979. Journal of Structural AYRTON, S. N. & RAMSAY,J. G. 1974. Tectonic and Geology, 1,338-9. metamorphic events in the Alps. Schweizerische DANIEL, T. 1986. Analyse de la ddformation des gran- Mineralogische und Petrographische ites: utilisation du traitement de l'image. Unpub- Mitteilungen, 54, 609-39. lished diploma thesis, Rennes University. BERTHE, D., CHOUKROUNE, P. & JEGOUZO, P. 1979. DAVY, P. & COBBOLD, P. R. (in press). Indentation Orthogneiss, mylonite and non coaxial defor- tectonics in nature and experiment. 1. Exper- mation of granites: the example of the South iments scaled for gravity. Bulletin of the Geo- Armorican Shear Zone. Journal of Structural logical Institutions of Uppsala, N. S., 14. Geology, 1, 31-42. DEUTSCH, A. & STEIGER, R. H. 1985. Hornblende BOCQUET, J., DELALAYE, M., HUNZIKER, J. C. & K-At ages and the climax of Tertiary metamor- KRUMMENACHER, D. 1974. K-Ar and Rb-Sr phism in the Lepontine Alps (south-central dating of blue amphibole, micas and associated Switzerland): an old problem reassessed. Earth minerals from the western Alps. Contributions to and Planetary Science Letters, 72, 175-89. Mineralogy and Petrology, 47, 7-26. ERNST, G. 1973. Interpretative synthesis of metamor- BRADBURY, H. J. & NOLEN-HOEKSEMA, M. C. 1985. phism in the Alps. Geological Society of America The Lepontine Alps as an evolving metamorphic Bulletin, 84, 2053-78. core complex during A-Type subduction: evi- ERNST, W. G. 1976. Mineralogic study of eclogitic dence from heat flow, mineral cooling ages and rocks from Alpe Arami, Lepontine Alps, tectonic model. Tectonics, 4, 187-211. southern Switzerland. Journal of Petrology, 18, CHADWICK, B. 1968. Deformation and metamorphism 371-98. in the Lukmanier region, central Switzerland. EVANS, B. W. & TROMMSDORF•, V. 1978. Petrogenesis Geological Society of American Bulletin, 79, of garnet lherzolite, Lima di Gagnone, Lepontine 1123-50. Alps. Earth and Planetary Science Letters, 40, CHOPIN, C. • MALLISKI, H. 1980. 39Ar-n~ dating 333-48. of high pressure metamorphic micas from the Fox, J. S. 1975. Three dimensional isograds from the Gran Paradiso area (western Alps): evidence Lukmanier pass, Switzerland, and their tectonic against the blocking temperature concept. Contri- significance. Geological Magazine, 112,547-64. butions to Mineralogy and Petrology, 74, 109-22. FRANK, E. 1983. Alpine metamorphism of calcareous & MONIE, P. 1984. A unique magnesiochlo- rocks along a cross-section in the central Alps: ritoid-bearing high-pressure assemblage from the occurrence and breakdown of muscovite, mar- Monte-Rosa, western Alps: petrologic and garite and paragonite. Schweizerische Min- 4~ radiometric study. Contributions to eralogische und Petrographische Mitteilungen, 63, Mineralogy and Petrology, 87, 388-98. 37-93. CHOUKROUNE, P. & GAPAIS, D. 1983. Strain pattern FRANK, M. & STETrLER, A. 1979. K-Ar and 39Ar/4~ in the Aar granite (central Alps): orthogneiss systematics of white K-mica from an alpine I32 O. Merle, P. R. Cobbold & S. Schmid

metamorphic profile in the . Schwei- Petrographische Mitteilungen, 46, 473-552.

zerische Mineralogische und Petrographische -- 1970. Polymetamorphism in the Monte Rosa, Mitteilungen, 59, 375-94. western Alps. Eclogae Geologicae Helvetiae, 63, FREY, M., HUNZIKER,J. C., FRANK, W., BOCQUET, J., 151-61. DAL PIAZ, G. V., J~,GER, E. and NIGGLI, E. HURFORD, A. & HUNZlKER, J. C. 1985. Alpine cool- 1974. Alpine metamorphism of the Alps. Sch- ing history of the Monte Mucrone eclogites weizerische Mineralogische und Petrographische (Sesia-Lanzo zone): fission track evidence. Sch- Mitteilungen, 54, 247-90. weizerische Mineralogische und Petrographische --, BUCHER, K., FRANK, E. & MULLIS, J. 1980. Mitteilungen, 65, 325-34. Alpine metamorphism along the geotraverse HURFORD, J. 1986. Cooling and uplift patterns in the Basel-Chiasso: a review. Eclogae Geologicae Lepontine Alps, south-central Switzerland, and Helvetiae, 73, 527-47. an age of vertical movement on the Insubric fault FUMASOLI, M. V. 1974. Geologie des Gebietes line. Contributions to Mineralogy and Petrology, n6rdlich und siidlich der Torio-Tonale-Linie 92, 413-27. im Westen yon Gravedona. Mitteilungen des J~,GER, E. 1973. Die Alpine Orogenese im Lichte der geologischen Instituts der Eidgen6ssischen Tech- Radiometrischen Alterbestimmung. Eciogae nischen Hochschule und der Universit~t Ziirich, Geologicae Helvetiae, 66, 11-21. neue Folge 194. , NIGGLI, E. & WENK, E. 1967. Rb/Sr Alter- GILBERT, E. and MERLE, O. 1987. Extrusion and bestimmungen an Glimmern der Zentralpen. radial spreading beyond a closing channel. Beitriige zur Geologischen Karte der Schweiz, Journal of Structural Geology, 9, 481-90. 134. GRECO, A. 1985. Analisi strutturale della parte front- KLAPER, E. M. 1982. Deformation und Metamor- ale del ricoprimento pennidico dell' Antigorio in phose in der n6rdlichen Maggia-zone. Schwei- Val Formazza (Novara, Italia). Schweizerische zerische Mineralogische und Petrographische Mineralogische und Petrographische Mitteilungen, Mitteilungen, 62, 47-76.

65, 299-23. -- & BUCHER-NURMINEN, K. 1987. Alpine meta- HALL, W. D. H. 1972. The structural geology and morphism of pelitic schists in the Nufenen pass metamorphic history of the Lower Pennine area, Lepontine Alps. Journal of Metamorphic nappes, Valle di Bosco, Ticino, Switzerland. Un- Geology, 5, 175-94. published PhD thesis, University of London. KLEIN, J. A. 1978. Post-nappe folding southeast of H~,NNY, R., GRAUERT, B. & SOPTRATANOVA, G. 1975. the MischabelriJckfalte (Pennine Alps) and some Paleozoic migmatites affected by high-grade aspects of the associated metamorphism. Leidse tertiary metamorphism in the central Alps (Valle Geologische Mededelingen, 51,233-312. Bodengo, Italy). Contributions to Mineralogy and KNUP, P. 1958. Geologie und Petrographie des Petrology, 51,173-96. Gebietes zwischen Centovalli, Valle Vigezzo und HEINRICH, C. A. 1982. Kyanite-eclogite to amphi- Onsernone. Schweizerische Mineralogische und bolite facies evolution of hydrous mafic and Petrographische Mitteilungen, 38, 217-232. pelitic-rocks, Adula nappe, central Alps. Contri- KI)PPEL, V. & GRONENFELDER, M. 1975. Concordant butions to Mineralogy and Petrology, 81, 30-38. U-Pb ages of monazite and xenotime from the

-- 1986. Eclogite facies regional metamorphism of central Alps and the timing of high temperature hydrous mafic rocks in the central Alpine Adula Alpine metamorphism, a preliminary report. nappe. Journal of Petrology, 27, 123-54. Schweizerische Mineralogische und Petrogra- HEITZMANN, P. 1987a. Calcite mylonites in the central phische Mitteilungen, 55, 129-32. Alpine 'root zone'. Tectonophysics, 135,207-15. & 1978. The significance of monazite U-Pb

-- 1987b. Evidence of late Oligocene/early Miocene ages: examples from the Lepontine area of the backthrusting in the central Alps. Geodynamica Swiss Alps. In: ZARTMAN, R. E. (ed). Fourth Acta, 1, 183-92. International Conference, Geochronology, Cos- HIGG1NS, A. K. 1964. The structure and metamorphic mochronology, Isotope Geology, 226-7. geology of the area between Nufenenpass and --, GONTHERT, A. & GRONENFELDER, M. 1980. Basodino, Ticino. Unpublished PhD thesis, Patterns of U-Pb zircon and monazite ages Imperial College, London. in polymetamorphic units of the Swiss central Hs0, K. J. 1979. Thin skinned plate tectonics during Alps. Schweizerische Mineralogische und Petro- neo-alpine orogenesis. American Journal of graphische Mitteilungen, 61, 97-119. Sciences, 279, 353-66. LAUBSCHER, H. P. 1971. The large scale kinematics of HUBER, M., RAMSAY, J. & SIMPSON, C. 1980. De- the western Alps and the northern Apennines formation in the Maggia and Antigorio nappes, and its palinspastic implications. American Lepontine Alps. Eclogae Geologicae Helvetiae, Journal of Science, 271,193-226.

73, 2, 593-606. -- 1983. The late Alpine (periadriatic) intrusions HUDLESTON, P. J. 1983. Strain patterns in an Ice cap and the Insubric Line. Memorie della Societ~ and implications for strain variations in shear Geologica Italiana, 26, 21-30. zones. Journal of Structural Geology, 5,455-63. MANCEL, P. & MERLE, O. 1987. Kinematics of the HUNZIKER, J. C. 1966. Zur Geologie und Geochemie northern part of the Simplon Line. Tectono- des Gebietes zwischen Valle Antigorio und Valle physics, 135, 265-75. di Campo. Schweizerische Mineralogische und MANCKTELOW, N. 1985. The Simplon Line: a major Lepontine dome - Tertiary kinematics I33

displacement zone in the western Lepontine di Padova, 30, 75-7. Alps. Eclogae Geologicae Helvetiae, 78, 73-96. SCHMID, S. M., ZING6, A. & HANDY, M. 1987. The 73-96. kinematics of movements along the Insubric Line 1987. Quartz textures from the Simplon fault and the emplacement of the Ivrea zone. Tectono- zone, southwest Switzerland and north physics, 135, 47-66. Italy. Tectonophysics, 135, 133-53. SCHMIDT, C. ~,~ PREISWERK, H. 1908. Geologische MARQUER, D. 8z GAPAIS, D. 1985. Les massifs cris- Karte der Simplongruppe. Spezialkarte n~ mit tallins externes sur une transversale Guttanen- Erliiuterungen. Val (Alpes centrales): structure et S~MPSON, C. 1982. The structure of the northern histoire cin6matique. Comptes-Rendus de l'Aca- lobe of the Maggia nappe, Ticino, Switzerland. d~mie des Sciences de Paris, 301, II, 543-6. Eclogae Geologicae Helvetiae, 75, 495-516. MAWER, C. K. 1987. Shear criteria in the Grenville -- & SCHMID, S. M. 1983. An evaluation of criteria Province, Ontario, Canada. Journal of Structural to deduce the sense of movement in sheared Geology, 9, 531-9. rocks. Geological Society of America Bulletin, MENARD, G. & THOUVENOT, F. 1984. Ecaillage de la 94, 1281-8. lithosphere europ6enne sous ies Alpes occiden- STAUB, R. 1924. Der Bau der Alpen: Versuch einer tales: arguments gravim6triques et sismiques li6s Synthese. Beitriige zur geologischen Karte der l'anomalie d'Ivr6a. Bulletin de la Soci~t~ G~o- Schweiz, NF 52. logique de France, XXVI, 875-84. STECK, A. 1984. Structures et d6formations tertiaires MERLE, O. 1987. Histoire de la d6formation dans les dans les Alpes centrales. Eclogae Geologicae Alpes lepontines occidentales (Alpes centrales). Helvetiae, 77, 55-100. Bulletin de la Soci~t~ Geologique de France, IIl, -- 1987. Le Massif du Simplon-r6flexions sur la 183-90. cin6matique des nappes de gneiss. Schweizeris- , LE GAL, PH. & MANCEL, P. 1986. D6formation che Mineralogische und Petrographische Mit- et m6tamorphisme dans la r6gion du Simplon teilungen, 67, 27-45. (Alpes centrales). Eclogae Geologicae Helvetiae, --, RAMSAY, J. G., MILNES, A. G. & BURRI, M. 79, 705-18. 1979. Compte-rendu de l'excursion de la Soci6te & 1988. Post-amphibolitic westward G6ologique Suisse et la Soci6t6 Suisse de Min- thrusting and fold vergence in the Ticino domain. 6ralogie et P6trographie en et en Italie Eclogae Geologicae Helvetiae, 81,215-36. Nord du 2 au 50ctobre 1978. Eclogae Geologicae MILNES, A. G. 1974a. Structure of the Pennine zone Helvetiae, 72, 287- 311. (central Alps): a new working hypothesis. Geo- STEIGER, R. H. 1983. Isotopic dating of metamorphic logical Society of America Bulletin, 85, 1727-32. events in the central Alps. Terra Cognita, 3, - 1974b. Post-nappe folding in the western Alps. p. 140. Eclogae Geologicae Helvetiae, 67, 2, 333-48. -- & BUCHER, I. 1978. Are Rb-Sr biotite ages in 1975. On steep isogradic surfaces in the Simplon the central Alps necessarily cooling ages? Fourth area: a discussion. Contributions to Mineralogy International Conference, Geochronology, Cos- & Petrology, 53, 65-8. mochronology, Isotope Geology, U.S. Geological 1978. Structural zones and continental collision, Survey, Open file report, 414-15. central Alps. Tectonophysics, 47, 364-92. S~EINER, H. 1984. Radiometrische Alterbestim- , GRELLER, M. & Mf.)LLER, R. 1981. Sequence mungen an Gesteinen der Maggia Decke and style of major post-nappe structures, (Penninikum der Zentralalpen). Schweizerische Simplon-Pennine Alps. Journal of Structural Mineralogische und Petrographische Mitteilun- Geology, 3, 411-20. gen, 64, 227- 59. MONIE, P. 1985. La m6thode 39Ar-4~ appliqu6e au STEINITZ, G. & J.~GER, E. 1981. Rb-Sr and K-Ar m6tamorphisme alpin dans le massif du Mont studies on rocks from the Suretta nappe, eastern Rose (Alpes occidentales). Chronologie d6taill6e Switzerland. Schweizerische Mineralogische und depuis 110 Ma. Eclogae Geologicae Helvetiae, Petrographische Mitteilungen, 61,121-31. 78, 487-516. STRECKEISEN,A. • WENK, E. 1974. On steep isogradic NI6CLI, E. 1970. Alpine Metamorphose und Alpine surfaces in the Simplon area. Contributions to Gebirgsbildung. Fortschritte der Mineralogie, 47, Mineralogy and Petrology, 47, 81-95. 16-26. THAKUR, J. C. 1973. Events in Alpine deformation PASSCHIER, C. W. & SIMPSON, C. 1986. Porphyroclast and metamorphism. Geologische Rundschau, 62, systems as kinematic indicators. Journal of Struc- 549-63. tural Geology, 8, 831-44. THOMPSON, P. H. 1976. lsograd patterns and pressure- PLATI', J. 1986. Dynamics of orogenic wedges and the temperature distributions during regional meta- uplift of high-pressure metamorphic rocks. Geo- morphism. Contributions to Mineralogy and logical Society of America Bulletin, 97, 1027-53. Petrology, 57, 277-95. PREISWERK. H. 1918. Geologische Beschreibung der TROMPY, R. 1980. An Outline of the Geology of Lepontinischen Alpen. Beitriige zur geologis- Switzerland. Wepf & Co, Basel. chen Karte der Schweiz, 26. TURNER F. J. 1968. Metamorphic Petrology. McGraw- PURDY, J. N. and J.~GER, E. 1976. K-Ar ages on rock Hill, New York. forming minerals from the central Alps. Memorie VAN DEN DRIESSCHE, J. & BRUN, J. P. 1987. Rolling Istituto di Geologia e Mineratogia dell' Universitf~ structures at large shear strain. Journal of Struc- I34 O. Merle, P. R. Cobbold & S. Schmid

tural Geology, 9, 691-704. che Mitteilungen, 35, 311 - 19. WAGNER, G. A., REIMER, G. M. & J,g,GER, E. 1977. WERNER, D., KOPPEL, V., H,Z,NNY, R. & RYBACH, L. Cooling ages derived by apatite fission track, 1976. Cooling models for the Lepontine area mica Rb-Sr and K-Ar dating: the uplift and (central Swiss Alps). Schweizerische Mineral- cooling history of the central Alps. Memorie ogische und Petrographische Mitteilungen, 56, lstituto di Geologia e Mineratogia dell' Universitd 661-7. di Padova, 30. ZINGG, A. t~ SCHMID, S. 1983. Mylonite zone along WENK, E. 1955. Eine Strukturkarte der Tessinenalpen. the Insubric Line west of the Lago Maggiore. Schweizerische Mineralogische und Petrographis- Terra Cognita, 3, NB 24.

O. MERLE & P. R. COBBOLD,Centre Armoricain d'Etude Structurale des Socles, CNRS, Univ. Rennes 1, 35042 Rennes Cedex, France. S. SCHMID, Geologisches lnstitut, ETH-Zentrum, 8092 Zurich, Switzerland.